Generally, in order to control hydraulic fluid that is supplied to a hydraulic actuator in controlling a discharge flow rate of a variable displacement main hydraulic pump, there is a positive control system (illustrated in FIG. 1(a)) that controls the discharge flow rate of the hydraulic pump in proportion to signal pressure that is input to a control valve and a negative control system (illustrated in FIG. 1(b)) that controls the discharge flow rate of the hydraulic pump in proportion to negative signal pressure that is generated on the downstream side of the control valve.
In the case of an engine that starts by a start motor, if the work place is located on high ground or in the winter season in which atmospheric temperature is low, the capability to appropriately combine and jet fuel and air to a cylinder is lowered due to lack of oxygen or reduction of pressure. Accordingly, the initial start of the engine may not be performed smoothly.
The startability is greatly affected by a rotating speed that is initially driven by a start motor. However, in designing the start motor, its capacity is constantly limited. In particular, in the case of an excavator, various hydraulic pumps connected to an engine PTO (Power Take Off) may act as resistance loads that lower the startability, and in this case, the engine may not start.
A negative control type hydraulic pump control device in the related art as illustrated in FIG. 2 includes a variable displacement hydraulic pump 1 connected to an engine (not illustrated), a control valve 2 installed in a bypass path 3 of the hydraulic pump 1 to control a flow rate discharged from the hydraulic pump 1 in accordance with an operation of an operation lever (that is, RCV (not illustrated)), a hydraulic actuator (not illustrated) such as a boom cylinder which is driven by hydraulic fluid that is supplied when the control valve 2 is shifted, an orifice 4 installed on a downstream side of the bypass path 3 to generate negative signal pressure, and a regulator 5 operated by the negative signal pressure to control discharge flow rate through control of an inclination angle of a swash plate of the hydraulic pump 1.
In the hydraulic pump control device as described above, if an operator does not operate the operation lever RCV, a spool of the control valve 2 is in a neutral state, and all hydraulic fluid that is discharge from the hydraulic pump 1 returns to a hydraulic tank T through the orifice 4. At this time, high pressure is formed in the orifice 4, and if the negative signal pressure is high as shown in FIG. 1(b), the regulator 5 controls the inclination angle of the swash plate of the hydraulic pump 1 so that the discharge flow rate of the hydraulic pump 1 returns to minimum capacity.
By contrast, if the operator shifts the control valve 2 by operating the operation lever RCV, the hydraulic fluid that is discharged from the hydraulic pump 1 is supplied to the hydraulic actuator. At this time, the flow rate that passes through the bypass path 3 is gradually reduced depending on the shift amount of the spool of the control valve 2. Accordingly, the negative signal pressure that is formed in the orifice 4 is reduced, and the regulator 5 controls the inclination angle of the swash plate of the hydraulic pump 1 so that the flow rate that can drive the hydraulic actuator is discharged.
In this case, if the engine is idle, no flow rate is supplied to the bypass path 3 even in the case where the spool of the control valve 2 is maintained in a neutral position. That is, since the inclination angle of the swash plate of the hydraulic pump 1 is positioned at the inclination angle of the swash plate whereby high flow rate is initially discharged, and thus relatively high starting torque is required at the initial start of the engine.
In the construction machine on which the negative control type hydraulic pump is installed as described above, if the engine start is stopped, the inclination angle of the swash plate of the hydraulic pump is stopped with the maximum capacity.
By contrast, if the engine starts in a state where the negative signal pressure is not formed, the inclination angle of the swash plate of the hydraulic pump is in the maximum capacity state. That is, if the flow rate is low and the negative signal pressure is not sufficient when the control valve, which generates negative signal pressure at the start of the engine, is in a neutral state, the inclination angle of the swash plate of the hydraulic pump is maintained with large capacity.
Further, in the low temperature condition in the winter season, since the viscosity of the hydraulic fluid becomes high, the agitating torque becomes high, and thus the torque that is consumed when the main hydraulic pump is rotated becomes relatively higher than that on the normal condition.